A. Symptoms and Etiology of Glaucoma
The term “glaucoma” refers generally to a group of diseases which cause progressive damage to the optic nerve and resultant optical field defects, vision loss and, in some cases, blindness. Glaucoma is frequently, but not always, accompanied by abnormally high intraocular pressure. Aqueous humor is continually produced by cells of the ciliary body and such aqueous humor fills the anterior chamber of the eye. Excess aqueous humor normally drains from the anterior chamber of the eye, through a structure known as the trabecular meshwork and then out of the eye through a series of drainage tubules. However, in many glaucoma patients, drainage of the aqueous humor through the trabecular meshwork is impaired, thereby causing the pressure of aqueous humor within the anterior chamber to increase.
In general, there are four types of glaucoma-primary, secondary, congenital and pigmentary. Primary glaucoma, which is the most common form, can be classified as either open angle or closed angle. Secondary glaucoma (e.g., neovascular glaucoma) occurs as a complication of a variety of other conditions, such as injury, inflammation, vascular disease and diabetes. Congenital glaucoma is elevated eye pressure present at birth due to a developmental defect in the eye's drainage mechanism. Pigmentary glaucoma is a rare form of the disease wherein pigment from the iris clogs the trabecular meshwork, preventing the drainage of aqueous humor from the anterior chamber.
Glaucoma is a leading cause of blindness in the United States. The loss of vision in glaucoma patients is typically progressive and may be due, at least in part, to compression of the vasculature of the retina and optic nerve as a result of increased intraocular pressure. It is generally accepted that reducing intraocular pressure, through the use of drugs and/or surgery, can significantly reduce glaucomatous progression in patients who suffer from normal-tension glaucoma and can virtually halt glaucomatous progression in patients who suffer from primary open-angle glaucoma with elevated intraocular pressures. Furthermore, it is generally acknowledged that lowering intraocular pressure in glaucoma patients can prevent or lessen the irreversible glaucoma-associated destruction of optic nerve fibers and the resultant irreversible vision loss.
B. Surgical Treatment of Glaucoma
The surgical treatment of glaucoma is generally aimed at either a) decreasing the amount of aqueous humor produced by the ciliary body or b) improving drainage of aqueous humor from the anterior chamber of the eye.
The procedures aimed at decreasing the production of aqueous humor include cyclocryotherapy, wherein a cryosurgical probe is used to freeze a portion of the ciliary body, thereby destroying cells that produce aqueous humor, and laser cyclophotocoagulation, wherein a laser is used to destroy part of the ciliary body resulting in decreased production of aqueous humor.
The procedures intended to improve drainage of aqueous humor from the anterior chamber include trabeculoplasty, trabeculectomy, goniectomy and shunt implantation.
In trabeculoplasty, the surgeon uses a laser to create small holes through the trabecular meshwork to increase aqueous humor drainage through the normal drainage channels.
In trabeculectomy, the surgeon removes a tiny piece of the wall of the eye, which may include a portion of the trabecular meshwork, thereby creating a new drainage channel which bypasses the trabecular meshwork and the normal drainage channels. Aqueous humor then drains with relative ease through the new drainage channel into a reservoir known as a “bleb” that has been created underneath the conjunctiva. Aqueous humor that drains into the bleb is then absorbed by the body. Trabeculectomy is often used in patients who have been unsuccessfully treated with trabeculoplasty or who suffer from advanced glaucoma where optic nerve damage is progressing and intraocular pressure is significantly elevated.
In goniectomy, a tissue cutting or ablating device is inserted into the anterior chamber of the eye and used to remove a full thickness strip of the tissue from the trabecular meshwork overlying Schlemm's canal. In many cases, a strip of about 2 mm to about 10 mm in length and about 50 μm to about 200 μm in width is removed. This creates a permanent opening in the trabecular meshwork through which aqueous humor may drain. The goniectomy procedure and certain prior art instruments useable to perform such procedure are described in U.S. patent application Ser. No. 10/052,473 published as No. 2002/0111608A1 (Baerveldt), the entirety of which is expressly incorporated herein by reference.
In shunt implantation procedures, a small drainage tube or shunt is implanted in the eye such that aqueous humor may drain from the anterior chamber, through the shunt and into a surgically created sub-conjunctival pocket or “bleb.” Aqueous humor that drains into the bleb is then absorbed by the patient's body.
Trabeculoplasty, trabeculectomy and shunt implantation procedures are sometimes unsuccessful due to scarring of closure of the surgically created channels or holes and/or clogging of the drainage tube. Because it involves removal of a full thickness strip from the trabecular meshwork, the goniectomy procedure is less likely to fail due to scarring or natural closure of the surgically created channel. Although the previously described devices can be used to successfully perform goniectomy procedures, there remains a need in the art for the development of new tissue cutting and ablation instruments that may be used to perform the goniectomy procedure as well as other procedures where it is desired to remove a strip of tissue from the body of a human or veterinary patient.